A variety of industrial processes produce water and wastewater streams that have relatively high concentrations of dissolved species. Accordingly, in many jurisdictions, regulations proscribe limits on the concentrations of particular species in wastewater, as well as on the total concentration of water born compounds in wastewaters. This latter criteria is often expressed as a limit on “total dissolved solids” (TDS). For example, in the United States, the Environmental Protection Agency has established National Secondary Drinking Water Regulations that set water quality standards, in the form of “secondary maximum contaminant levels”, for drinking water. These include a guideline maximum sulphate concentration of 250 mg/L and a maximum TDS of 500 mg/L. Other regulations in the United States proscribe limits on the TDS of fresh water to be used for agriculture of 1,000 mg/L and a TDS limit of 1,500 mg/L for fresh water to be used in fresh water aquaculture.
Acid mine drainage constitutes one category of wastewater that often requires treatment in order to meet regulatory discharge standards. For example, a variety of neutralization processes may be used for treating acidic mine drainage, using limestone (CaCO3), hydrated lime (Ca(OH)2) and/or quicklime (CaO) as neutralization agents. In these lime treatment processes, sufficient alkalinity is typically added to raise pH and thereby to form insoluble metal hydroxides that settle out of the water while the predominant anion, sulphate, precipitates as gypsum (CaSO4*2H2O) or gypsum anhydrite (CaSO4). While drastically reducing the concentration of some species, particularly heavy metals, these processes may produce wastewaters that have very high residual calcium and/or magnesium cation concentrations, i.e. hard water, as well as high concentrations of sulphate anions. The effluent dissolved calcium and sulphate concentrations are controlled by the solubility of the gypsum species, which is theoretically approximately 2.6 g/L CaSO4*2H2O but which varies considerably depending on the other ions in the water and on the concentrations of ions fed to the lime treatment process. Effluents from these lime neutralization plants may, for example, be characterized by the following parameters: pH 6 to 10.5; SO42− 1,000 to 2,200 mg/L; Ca2+ 400 to 800 mg/L; and Mg2+ 0 to 500 mg/L. Other anions such as NO3−, Cl−, and HCO3− may for example be present in the range of 0 to 500 mg/L. Other cations, such as Na+, K+, NH4+, may for example be present in the range of 0 to 500 mg/L. Bleed streams from flue gas desulphurization scrubbing circuits may also produce aqueous effluents that are relatively hard, with Mg2+ of 3500 mg/L, Ca2+ of 400 to 2000 mg/L, and SO42− of 1000 to 5000 mg/L. There may accordingly be a need for further treatment, following lime treatment, of these waters to meet particular discharge, or re-use, requirements.
A very wide variety of processes have been used to remove ionic species from water, primarily for the treatment of industrial wastewaters and the purification of drinking water. Cation and anion exchange resins have for example been used together in circuits adapted for the treatment of mine waters high in calcium and sulphate (see: U.S. Pat. No. 5,269,936; International Patent Publication No. WO/1998/058737; Everett, D. J., Du Plessis, J. & Gussman, H. W. (1993): The Treatment of Underground Mine Waters for the Removal of Calcium and Sulphates by a GYP-CIX Process.—In: International Mine Water Association & Zambia Consolidated Copper Mines Limited: The First African Symposium on Mine Drainage and Environment Protection from Mine Waste Water Disposal.—p. 463-491; Chililabombwe (Konkola Division); The treatment of acid effluent from the Grootvlei Mine using novel IX techniques. Robinson, R. E. Barnard, R. Le Riche, F. J., JOURNAL-SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY 1998, VOL 98; NUMBER 7, pages 343-352. Conventionally, sulphuric acid and lime are used to regenerate the cation and anion exchange resins in these processes, to produce gypsum (calcium sulfate dihydrate, CaSO4.2H2O) as a solid by-product of resin regeneration. The cost of the regeneration process inputs, as well as the costs of dealing with the associated regeneration products, may represent a significant proportion of the total operating costs of such processes. Anion exchange resins have also been used, without a preceding step of cation removal, to soften water by first removing anions, such as sulphate, with a concomitant increase in pH that is utilized to precipitate calcium carbonate (U.S. Pat. No. 6,059,974). Carbon dioxide may be used in this process, to facilitate calcium carbonate precipitation. Carbon dioxide is soluble in water, with which it reacts to form a balance of several ionic and non-ionic species: dissolved free carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate anions (HCO3−) and carbonate anions (CO32−), in equilibrium as follows:CO2 (aq)+H2OH2CO3HCO3−+H+CO32−+2H+A high pH will push this equilibrium towards carbonate formation, and hence facilitate the precipitation of calcium carbonate.